1. Field of the Invention
This invention relates to a booster circuit and more particularly to a pump-type booster circuit.
2. Description of the Related Art
When it is required to use a voltage higher than the power supply voltage in a semiconductor circuit, the required voltage is generally acquired by use of a pump-type booster circuit and a control circuit thereof. A comparator (control circuit) compares the output voltage of the booster circuit with a target voltage, operates the booster circuit if the output voltage is lower than the target voltage and interrupts the operation of the booster circuit if the output voltage is higher than the target voltage. The pump-type booster circuit discharges a preset amount of charge for each pumping operation to repeatedly perform the pumping operation until the target voltage is exceeded. The current driving ability of the booster circuit is generally designed to attain the target output voltage even in a case where the load current is maximum.
In Jpn. Pat. Appln. KOKAI Publication No. 2001-250381, it is disclosed that a plurality of booster circuits having different charge supply abilities are provided and one of the booster circuits is selected and operated according to the operation of a memory circuit which is supplied with the boosted voltage output from the booster circuit. That is, for example, a first booster circuit is operated in synchronism with activation of the precharge signal of the memory circuit and a second booster circuit is operated in synchronism with activation of a word line. U.S. Patent Specification No. 6,459,643B2 corresponds to the U.S. patent specification of the KOKAI publication and has the same contents.
In Jpn. Pat. Appln. KOKAI Publication No. 2000-350439, it is disclosed that a plurality of booster cell groups are provided and the connection state of series connection and parallel connection of the plurality of cell groups is selectively switched according to a control signal from the exterior.